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communicating between two frames from different servers?

I'm trying to send some form values via JS from one frame (a PHP application I control which creates form fields from database values) to another (a shopping cart on another site) - but I can't do this because normal scripts are barred from setting values on pages from other servers... unless the script is signed.

So, am I right that I HAVE to use a signed script to accomplish this goal? Are there lots of cross-platforms issues in getting extra privileges for signed scripts? (Does IE even recogize signatures the way NN does?)
I've never worked with signed scripts before - are they worth the hassle?